Airway model
In this chapter we will model how changes in airway structure affect the resistance to flow. For the purpose of modelling the intrathoracic or lower airways can be schematically depicted as structures with the following components:
- Cartilage
- Smooth muscles
- Mucosa, submucosa including vessels
- Alveolar attachments
Consult the animation on the left for further information. Shortly we will go into the role of lung elastic recoil.
Physiological model
To better understand the mechanical behavior of airways it helps
to model the various constituents within the airway wall. We
can model the airway as a circular structure with an inner and
an outer diameter. The airway wall thickness can be increased
internal to the smooth muscle layer due to inflammatory changes:
cellular infiltration, hypertrophy, hyperplasia, edema, vascular
engorgement, and by the accumulation of mucus at the inner surface
of the airway. Increased thickness of the airway wall is usually
due to smaller internal diameter and hence associated with increased
airways resistance. However, outer airway diameter may also increase
due to swelling of the adventitia external to the smooth muscle
layer.
The alveolar attachments confer stability to the airway wall as
they pull the airway wall outwards. When the airway muscle contracts,
the muscles are loaded by the elastic recoil of the surrounding
tissues, or the subatmospheric pleural pressure, which exert a tethering
force on intrapulmonary airways. Thus the surrounding structures
can be depicted as springs inserting on the outer airway wall.